I. Field of the Invention
The present invention relates generally to satellite communication systems, and more particularly, to an apparatus, system, and method for providing an audio interface between multiple deskset phones and a radio transceiver unit.
II. Description of the Related Art
A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users. However, spread spectrum modulation techniques, such as code division multiple access (CDMA) spread spectrum techniques, provide significant advantages over other modulation schemes, especially when providing service for a large number of communication system users. The use of CDMA techniques in multiple access communication systems is disclosed in U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990, entitled Spread Spectrum Multiple Access Communication. System Using Satellite Or Terrestrial Repeaters, and U.S. Pat. 5,691,974, which issued Nov. 25, 1997, entitled Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy, both of which are assigned to the assignee of the present invention, and incorporated herein by reference. CDMA uses a preselected pseudonoise (PN) code sequence to modulate and spread a digital message over a predetermined spectral band, prior to modulation of the carrier signal. The same PN code is used to recover the original digital message at the destination.
The above referenced patents disclose communication systems in which a large number of generally mobile or remote system users employ mobile stations or subscriber units (xe2x80x9cuser terminalsxe2x80x9d) having at least one transceiver to communicate with other user terminals, or users of other connected systems, such as a public telephone switching network. Communication signals are transferred either through satellites and gateways, or directly to terrestrial base stations (also sometimes referred to as cell-sites or cells).
One type of remote user is a fixed unit, such as a wireless phone, facsimile device, and so forth, in a remote location where wirelines are impractical, such as an offshore oil rig or other remote geographical location. Such remote locations often require that multiple phones be serviced by a single access channel or communication link from a satellite in a xe2x80x9cparty linexe2x80x9d type of service. A party line is a single communications path or link that is used as a shared resource. The defining feature of party line service is that a telephone call can be answered or initiated by a user at one party line phone, and all other party line phones can participate in the telephone call. But, an additional telephone call by a non-participating phone cannot be effected until the first call is terminated.
An exemplary situation would be a telephone call between workers on an offshore oil rig and land-based vendor technicians, where the purpose is to solve a technical problem. It would be advantageous for workers at multiple locations on the oil rig to be able to simultaneously participate in the telephone call. A party line, generally having a single access, address, code, or telephone number, can meet this need.
Another situation in which multiple phones may be required is in a remote geographical land location where it is not cost effective to run standard telephone lines, including mid-desert locations, small island locations, rural-third world locations, and the like. In those cases, a community building having multiple rooms might be setup with phones in each room. Another example would be a remotely located multi-family dwelling, where each family would want to have a phone. In many of these cases, it would not be cost effective to install separate satellite receiving equipment for maintaining separate communications links for each phone. In each of these cases, a party line arrangement would provide a cost effective method of maintaining efficient telephone communication links.
A remote location with party line service can be efficiently linked to a satellite communications system through a radio antenna unit (RAU). An RAU is a transceiver, comprising well known elements, that transmits and receives a modulated carrier signal to and from the satellite communications system through an antenna. During transmission, the RAU accepts audio signals from multiple phones. An audio coder-decoder (or audio codec) in the RAU digitizes the audio signals, which are then used to modulate the carrier signal that is radiated to a satellite (or other relay apparatus) by the antenna. During reception, the RAU receives an input signal comprising a modulated carrier signal from a satellite. The RAU demodulates the input signal to retrieve the digital audio signal. After which, the audio codec converts the digital audio signal to an analog audio signal, and causes the analog audio signal to be sent to the multiple phones. The RAU can use a variety of additional signal processing and control elements as desired, and known in the art.
When the audio signals are primarily composed of human speech, a vocoder may be used to compress (de-compress) the digital bit stream before the carrier signal is modulated (de-modulated) to make more efficient use of the carrier signal bandwidth. The vocoder operates on the principle that speech sounds can be predicted and extrapolated based on the analysis of a small portion of a sound. Thus, the vocoder removes selected bits from the digital bit stream before carrier modulation, and adds them back before de-modulation. Vocoders are especially useful in wireless communications systems where multiple subscribers are competing for limited carrier bandwidth.
The above described communications system requires an audio interface to carry the electrical audio signals between the multiple phones and the RAU.
A conventional audio interface for a POTS (Plain Old Telephone Service) system is full duplex, meaning outgoing transmit signals and incoming receive signals are both carried on the same cable simultaneously. However, both the vocoder and the audio codec require that the transmit and receive signals be separated for proper operation. A POTS system utilizes a hybrid transformer located at either the telephone switching office or in the phone handset to separate the transmit and receive signals.
Interactions between the hybrid transformer, and vocoder or audio codec result in an echo signal that is reflected back to the phone user. Loud echo signals degrade reception quality and are generally unacceptable. A conventional POTS system implements elaborate echo cancellation circuitry at the telephone switching office to suppress or attenuate the echo signal so that it is not reflected back to the phone user. The echo cancellation circuitry is expensive, but since a typical POTS telephone office serves a large number of users, the expense is justified.
The conventional POTS audio interface is an inappropriate choice for a remote location served by a single RAU because it would require the installation of the echo cancellation circuitry at the RAU. Although the RAU supports multiple phones, the number of phones is typically insufficient to justify the expense of the echo cancellation circuitry. What is needed is an audio interface between multiple phones and a single RAU that does not require the use of echo cancellation circuitry.
The present invention is directed toward an apparatus, system, and method for providing an audio interface between a deskset phone at a first location and an audio codec at a second location. In one embodiment the audio codec is a leading component in a radio antenna unit (RAU).
The audio interface comprises a transmit path comprising a 2-wire transmit cable for carrying a transmit signal from the first location to the second location, and a receive path comprising a 2-wire receive cable for carrying a receive signal from the second location to the first location. The transmit and receive paths are electrically isolated from each other, except that the receive signal contains an intentional sidetone that is derived from the transmit signal. In one embodiment, the transmit and receive cables are twisted together to further the goal of electrical isolation.
In one embodiment, the transmit path further comprises a bias circuit that biases a microphone at the first location with a floating point ground causing the transmit signal to be a differential signal. The bias circuit also converts any single mode power supply noise to common mode noise. A first amplifier, configured with a differential input and output, amplifies the transmit signal to drive the transmit cable. Since the first amplifier has a differential input, it rejects any common mode noise added to the transmit signal by the microphone bias circuit. A transformer attached to the transmit cable at the RAU location rejects any common mode noise generated by the transmit cable, but passes the transmit signal.
In an alternate embodiment, the audio interface provides an interface between multiple deskset phones and the audio codec. A summing amplifier combines multiple transmit signals from multiple deskset phones to form a combined transmit signal. The summing amplifier comprises: the transmit cable for summing currents of the multiple transmit signals; an amplifier for amplifying the combined transmit signal; and a multiple series resistors that correspond to the multiple desksets. The value of the series resistors determines the relative weighting of each transmit signal in the combined transmit signal.
In one embodiment, the receive path further comprises a summing amplifier. The receive summing amplifier combines a primary receive signal and a sidetone signal to form the receive signal, and amplifies the receive signal. A transformer attached to the receive cable at the deskset location rejects any common mode noise picked up by the receive cable, but passes the receive signal.
In an alternate embodiment, DC power is generated at the RAU location and carried by the transmit and receive cables to the deskset locations. Transformers attached to both ends of the transmit and receive cables isolate individual power supply voltages from the audio circuitry at the deskset and the RAU locations.
One main advantage of the present invention is that it allows for the cost effective installation and operation of multi-party telephone and data communications services in remote locations where standard telephone communications lines cannot be installed at a reasonable cost.
A second advantage of the present invention is that the audio interface contains a transmit cable and a receive cable. The transmit cable is dedicated to carrying transmit signals and is electrically isolated from the receive cable that is dedicated to carrying receive signals. This results in cost saving and reduced system complexity over a conventional POTS audio interface.
A conventional POTS interface requires a hybrid transformer to separate the transmit and receive signals prior to the audio codec because the transmit and receive signals are carried on the same cable simultaneously. Furthermore, a conventional POTS interface requires expensive echo cancellation circuitry to cancel the echo signal caused by the hybrid transformer. The present invention does not require a hybrid transformer or the echo cancellation circuitry because the present invention utilizes separate cables for the transmit and receive signals.
A third advantage of the present invention is that the components in the transmit and receive paths are configured to operate on differential signals, and reject any common mode signals. This is advantageous for reflecting common mode noise that can saturate audio components if unchecked. The transmit and receive cables are large potential sources of common mode noise, where the noise power generated increases with cable length. The present invention utilizes audio transformers attached to the ends of the transmit and receive cables to reject any common mode noise generated by the cables.
A fourth advantage is the use of a microphone bias circuit that converts single mode power supply noise to common mode noise that is imposed on the differential transmit signal. As discussed above, the common mode noise is then rejected by following audio components because they are configured to operate on differential signals only.
A fifth advantage of the present invention is that the transmit and receive cables can be utilized to carry DC power generated at the RAU location to the deskset phones. Thus, one DC supply can power the RAU and deskset components, without the need to run a separate set of power cables between the different locations. Audio transformers attached to the ends of the transmit and receive cables prevent the cable voltage from affecting the attached audio circuitry.